As economy develops, more and more energy is demanded. Amidst the continuous growth of energy demand, energy source is still dependent on fossil fuels such as coal and petroleum. However, the harmful products or pollutants generated by the burning of fossil fuels are responsible for global pollution. To prevent the release of pollutants into the atmosphere and to stop the pollution of global environment, development work is being carried out at an accelerated pace to create a flue gas treatment system for installation in fuel combustion plant such as thermal power plants. There are still many areas of improvement to meet problems such as the complicated configuration of the equipment requiring a large number of control variables and the need for large-scale waste water treatment systems requiring sophisticated treatment technology.
In an effort to solve these problems, a flue gas treatment system in which flue gas discharged from the fuel combustion facility such as a boiler is treated by irradiation of electron beam has been developed.
In this system, ammonia is injected into a high temperature gas containing sulfur oxides, and the mixed gas is irradiated with electron beam to remove the sulfur oxides therefrom in the form of a powder of ammonium compounds. In this case, the lower the temperature of gas is, the higher the reaction rate between sulfur oxides and ammonia is. Therefore, the high temperature gas is required to be cooled to a certain range of temperature. Accordingly, conventionally, the gas is normally cooled to a temperature ranging from an adiabatic saturation temperature plus 10.degree. C. to 80.degree. C. by water spray at the top of a cooling tower which is located in the system upstream of a process vessel in which ammonia is injected. The cooling tower is a complete evaporating type in which the sprayed water is completely evaporated. The gas cooling system has such an advantage that no waste water treatment system is required to be installed because of generation of no waste water. Further, by controlling the amount of sprayed water, the temperature of gas discharged from the cooling tower can be adjusted.
However, if the concentration of sulfur oxides is relatively large and/or the dose of electron beam is relatively large, an increase of the gas temperature caused by the heat of reaction between sulfur oxides and ammonia and/or by the heat generation due to irradiation of electron beam is not negligible. Therefore, in order to avoid lowering the reaction rate due to an increase of the gas temperature, after cooling of the gas in the cooling tower, it is necessary to adjust the gas temperature in the range of 50 to 80.degree. C. by water spray in a process vessel. At this time, the sprayed water is completely evaporated in the process vessel or in the subsequent stage, and hence waste water is not generated in the process vessel as well (hereinafter suppression of an increase of the gas temperature by evaporation of sprayed water in the process vessel is referred to as "secondary gas cooling", and the cooling of the gas before the reaction step is referred to as "primary gas cooling").
FIG. 3 is a schematic view of a conventional electron beam flue gas treatment system. As shown in FIG. 3, flue gas containing sulfur oxides discharged from a boiler 1 which is a kind of fuel combustion facility is cooled in a heat exchanger 2, and then introduced into a cooling tower 4. In the cooling tower 4, water supplied from a pump 3 is sprayed by a single-fluid nozzle 6, and the sprayed water is completely evaporated therein. The cooling tower 4 is a complete evaporating type in which the sprayed water is completely evaporated. The flue gas is cooled to a certain range of temperature in the cooling tower 4, and then the cooled gas is introduced into a process vessel 5.
On the other hand, ammonia supplied from an ammonia supply equipment 9 is mixed with air in a line mixer 10. The mixed gas and water supplied from a water supply source (not shown) are mixed in a gas-liquid mixing room of a two-fluid nozzle 11, and sprayed at the entrance of the process vessel 5. The mixture of the gas and water are irradiated with electron beam from an electron accelerator 12.
According to the conventional method in which the primary gas cooling is conducted in a complete evaporating type cooling tower, a gas retention time of 10 to 30 seconds in the cooling tower is required to evaporate the sprayed water completely in the cooling tower. This causes the problems that a large volume cooling tower is needed, and a high construction cost of the cooling tower and a large space for the cooling tower are required. It is possible to decrease the volume of the cooling tower by decreasing the diameter of the droplets of the sprayed water. However, this method is problematic in that more power is required for atomization of the water into fine droplets.